There are plans to improve the board manager but there are some things that might help understand more:-

1) In recent months the size of many board packages has changed from 2 or 300mb to 2 or 3GB. They are huge. Therefore, the time taken to download depends on which packages you are inbstalling.

2) Over the past year or so a new compression type is often used by the packages. It gives a small download size but takes longer to unpack. That is heavily depedant on cpu and disk speed.

3) It is possible to continue using the explorer to download multiple packages at a time. The status bar of the explorer shows "Downloading or unpacking 'n' pcakges". When all packages are downloaded and expanded the status changes to "Click the reaload button to use the new packages"

4) It is possible to open a 2nd IDE instance and install from that one whilst using the other instance. In this case, click "vMicro>general>reload toolchains" to use the newly installed packages in the ide used for dev or restart it.

5) The board and library manager are compatible with the arduino ide. That can also be used to install. The actions in 4) above apply after installing via arduino ide.

6) Use "Task Manager > Performance" to view nbetwork, cpu or disk to see what is taking the time. It will either be the download or the unpacking (cpu)

7) Note. Uploading or building with a toolchain that is about to be replaced by a downloaded newer version locks the .exe files which prevents windows from deleting the folder prior to the new version being applied. This can cause a mess in the arduino packages folder where older toolchains are not deleted properly. Doesn't affect operation but can consume disk space.

8) There is little visual micro code involved in the download and install of new board packages. It's simply download and unpack.

I hope that helps.